The present invention relates to coated abrasives and particularly to abrasive products adapted to perform in an improved fashion when used under moderate to low pressure grinding conditions.
In the production of coated abrasives a backing material, which may be treated to modify the absorptive properties, is provided with a make coat comprising a curable binder resin and abrasive grains are deposited on the make coat before the binder is at least partially cured. Thereafter a size coat comprising a curable binder resin is deposited over the abrasive grain to ensure the grains are securely anchored to the backing.
When the coated abrasive is used to abrade a workpiece, the tips of the abrasive grains lying in the plane of the surface contact the workpiece and begin the work of abrasion. The grains thus contacting the workpiece are subjected to great stresses and, if the grain is not adequately held by the size coat it can be plucked from the surface before it has completed abrading. The bond therefore should hold the grain securely. As abrading continues the grain can become polished, in which case significant frictional heat is generated and little removal of the workpiece occurs. In addition the stresses build up further and eventually the grain is either plucked out completely of it fractures so that a large portion is lost. This however generates new sharp edges such that abrading can resume. Ideally the mode of fracture should be as small as possible such that each grain will last a long time. This is achieved using sol-gel alumina abrasive grains which each comprises micron-sized or smaller crystallites which, under grinding conditions, can break off to reveal new cutting edges. However this occurs under moderate to heavy grinding pressure and only a reduced amount of self-sharpening occurs at lower pressure grinding conditions. There is therefore a need for a highly effective abrasive particle that operates very efficiently at moderate to low pressure grinding conditions.
One option that has been explored is the use of agglomerated abrasive grains in which an abrasive particle made up of a number of finer abrasive particles is held together by a bond material that can be organic or vitreous in nature. Because the bond is in general more friable than the abrasive particles, the bond fractures under grinding conditions that would otherwise lead to polishing or wholesale fracture of the abrasive grain.
Agglomerated abrasive grain generally permit the use of smaller particle (grit) sizes to achieve the same grinding efficiency as a larger abrasive grit size. Agglomerated abrasive grains have also been reported to improve grinding efficiency.
U.S. Pat. No. 2,194,472 to Jackson discloses coated abrasive tools made with agglomerates of a plurality of relatively fine abrasive grain and any of the bonds normally used in coated or bonded abrasive tools. Organic bonds are used to adhere the agglomerates to the backing of the coated abrasives. The agglomerates lend an open-coat face to coated abrasives made with relatively fine grain. The coated abrasives made with the agglomerates in place of individual abrasive grains are characterized as being relatively fast cutting, long-lived and suitable for preparing a fine surface finish quality in the work-piece.
U.S. Pat. No. 2,216,728 to Benner discloses abrasive grain/bond agglomerates made from any type of bond. The object of the agglomerates is to achieve very dense wheel structures for retaining diamond or CBN grain during grinding operations. If the agglomerates are made with a porous structure, then it is for the purpose of allowing the inter-agglomerate bond materials to flow into the pores of the agglomerates and fully densify the structure during firing. The agglomerates allow the use of abrasive grain fines otherwise lost in production.
U.S. Pat. No. 3,048,482 to Hurst discloses shaped abrasive micro-segments of agglomerated abrasive grains and organic bond materials in the form of pyramids or other tapered shapes. The shaped abrasive micro-segments are adhered to a fibrous backing and used to make coated abrasives and to line the surface of thin grinding wheels. The invention is characterized as yielding a longer cutting life, controlled flexibility of the tool, high strength and speed safety, resilient action and highly efficient cutting action relative to tools made without agglomerated abrasive grain micro-segments.
U.S. Pat. No. 3,982,359 to Elbel teaches the formation of resin bond and abrasive grain agglomerates having a hardness greater than that of the resin bond used to bond the agglomerates within an abrasive tool. Faster grinding rates and longer tool life are achieved in rubber bonded wheels containing the agglomerates.
U.S. Pat. No. 4,355,489 to Heyer discloses an abrasive article (wheel, disc, belt, sheet, block and the like) made of a matrix of undulated filaments bonded together at points of manual contact and abrasive agglomerates, having a void volume of about 70-97%. The agglomerates may be made with vitrified or resin bonds and any abrasive grain.
U.S. Pat. No. 4,364,746 to Bitzer discloses abrasive tools comprising different abrasive agglomerates having different strengths. The agglomerates are made from abrasive grain and resin binders, and may contain other materials, such as chopped fibers, for added strength or hardness.
U.S. Pat. No. 4,393,021 to Eisenberg, et al, discloses a method for making abrasive agglomerates from abrasive grain and a resin binder utilizing a sieve web and rolling a paste of the grain and binder through the web to make worm-like extrusions. The extrusions are hardened by heating and then crushed to form agglomerates.
U.S. Pat. No. 4,799,939 to Bloecher teaches erodable agglomerates of abrasive grain, hollow bodies and organic binder and the use of these agglomerates in coated abrasives and bonded abrasives. Higher stock removal, extended life and utility in wet grinding conditions are claimed for abrasive articles comprising the agglomerates. The agglomerates are preferably 150-3,000 microns in largest dimension. To make the agglomerates, the hollow bodies, grain, binder and water are mixed as a slurry, the slurry is solidified by heat or radiation to remove the water, and the solid mixture is crushed in a jaw or roll crusher and screened.
U.S. Pat. No. 5,129,189 to Wetscher discloses abrasive tools having a resin bond matrix containing conglomerates of abrasive grain and resin and filler material, such as cryolite.
U.S. Pat. No. 5,651,729 to Benguerel teaches a grinding wheel having a core and an abrasive rim made from a resin bond and crushed agglomerates of diamond or CBN abrasive grain with a metal or ceramic bond. The stated benefits of the wheels made with the agglomerates include high chip clearance spaces, high wear resistance, self-sharpening characteristics, high mechanical resistance of the wheel and the ability to directly bond the abrasive rim to the core of the wheel. In one embodiment, used diamond or CBN bonded grinding rims are crushed to a size of 0.2 to 3 mm to form the agglomerates.
U.S. Pat. No. 4,311,489 to Kressner discloses agglomerates of fine (xe2x89xa6200 micron) abrasive grain and cryolite, optionally with a silicate binder, and their use in making coated abrasive tools.
U.S. Pat. No. 4,541,842 to Rostoker discloses coated abrasives and abrasive wheels made with agglomerates of abrasive grain and a foam made from a mixture of vitrified bond materials with other raw materials, such as carbon black or carbonates, suitable for foaming during firing of the agglomerates. The agglomerate xe2x80x9cpelletsxe2x80x9d contain a larger percentage of bond than grain on a volume percentage basis. Pellets used to make abrasive wheels are sintered at 900xc2x0 C. (to a density of 70 lbs/cu. ft.; 1.134 g/cc) and the vitrified bond used to make the wheel is fired at 880xc2x0 C. Wheels made with 16 volume % pellets performed in grinding with an efficiency similar to that of comparative wheels made with 46 volume % abrasive grain. The pellets contain open cells within the vitrified bond matrix, with the relative smaller abrasive grains clustered around the perimeter of the open cells. A rotary kiln is mentioned for firing the green foam agglomerates.
U.S. Pat. No. 5,975,988 teaches conventional abrasive agglomerates comprising abrasive particles dispersed in a binder matrix but in the form of shaped grains deposited in a precise order on a backing and bonded thereto.
U.S. Pat. No. 6,319,108 describes a rigid backing with, adhered thereto by a metal coating, a plurality of abrasive composites comprising a plurality of abrasive particles dispersed throughout a porous ceramic matrix.
None of these prior art developments suggest the manufacture of coated abrasives using porous agglomerated abrasive grain as the term is used herein and a bond. Neither do they suggest the production of a product with abrasive particles held together by a relatively small amount of bond such that the particle binder phase is discontinuous. The methods and tools of the invention yield new structures and benefits from the use of such agglomerated abrasive grains, yet they are sophisticated in permitting the controlled design and manufacture of broad ranges of abrasive article structures having beneficial interconnected porosity characteristics. Such interconnected porosity enhances abrasive tool performance in large contact area, precision grinding operations, and in general relatively medium to low pressure grinding applications.
The present invention provides a coated abrasive article comprising a backing material and adhered thereto by a binder material, abrasive agglomerate grains characterized in that the grains used in the production of the coated abrasive comprise a plurality of abrasive particles adhered together in a three dimensional structure in which each particle is joined to at least one adjacent particle by a particle binder material which is present in the agglomerate as a discontinuous phase within the agglomerate grain and is located essentially completely in the form of bond posts linking adjacent particles, such that the agglomerate has a loose pack volume that is at least 2% lower than that of the abrasive particles in the individual state.
In this application the term xe2x80x9cgrainsxe2x80x9d will be reserved for agglomerates of a plurality of abrasive xe2x80x9cparticlesxe2x80x9d. Thus the grains will have the above identified porosity characteristics whereas the particles will have essentially zero porosity. Further the binder holding the particles together is identified as a xe2x80x9cparticle binderxe2x80x9d which may be the same, (or more often different from), the binder by which the grains are attached to the backing material.
The particle binder in the agglomerate grains is located essentially completely in the form of bond posts and this means that at least 70% of the binder, and preferably in excess of 80%, is used to form bond posts linking adjacent particles. A bond post is formed under agglomerate forming conditions when the particle binder is in a fluid condition and tends first to coat the particles and then to flow to points of contact or closest approach of adjacent particles and to merge with the binder associated with such adjacent particles. When the temperature is lowered and the binder solidifies the binder forms a solid contact between the particles that is known as a xe2x80x9cbond postxe2x80x9d. Naturally each bond post is also attached to the surface of the particles it connects but this binder is considered part of the bond post for the sake of this description. This does not exclude the possibility that some relatively small amount is present as a coating on at least part of the particle surface not associated with a bond post. It is intended however to exclude the situation in which the particles are embedded in a matrix of binder as occurs in conventional aggregate abrasive grains. As is apparent from examination of FIGS. 5-7 of the Drawings the individual abrasive particles making up the agglomerate grain are individually identifiable and indeed are essentially all that can be seen in typical agglomerate grains according to the invention. It is therefore possible to describe the particles as being xe2x80x9cagglomeratedxe2x80x9d implying being linked together rather than being held in a matrix which fills the larger portion of the space between the particles. Naturally when larger numbers of particles are agglomerated some individuals within the agglomerate will not be individually visible, but if it were possible to take a cross-section, the same pattern of individual particle visibility would be evident.
Clearly when the number of particles agglomerated becomes large, there will necessarily be substantial volumes of porosity created by this agglomeration. This can be as much as 70% of the total apparent volume of the agglomerate. However when the numbers of particles agglomerated are small, perhaps in the single figures, the concept of xe2x80x9cporosityxe2x80x9d becomes less useful in describing the agglomerates. Examples of such agglomerates showing the kind of structures involved are illustrated in FIGS. 5-7.
For this reason the term xe2x80x9cloose pack volumexe2x80x9d (LPV) is adopted. The LPV value is obtained by dividing the solid volume, (that is the total actual volume of the solids in the abrasive grain or particle, including the bond component) by the apparent volume of the agglomerate grain. The highest possible figure will be obtained from the particles themselves without any agglomeration having taken place. The larger the number of particles agglomerated, the greater the divergence from the maximum figure. Thus while the difference can be as low as 2% it can rise to 40% or even higher when larger numbers of particles are agglomerated together in the manner taught herein.
The calculation of the LPV is exemplified using the following data which represent actual agglomerate made using 60 grit particles of a seeded sol-gel alumina as the abrasive particles and a conventional vitreous bond suitable for use with such particles using a process substantially as described in Example 2 below.
The products are identified by the agglomerate grain size shown at the head of each column. In each case the measurements were made of the basis of a fixed volume of the agglomerate abrasive grains, referred to here as the xe2x80x9cApparent Volumexe2x80x9d.
As will be appreciated from the above, the larger the agglomerate grain, the smaller the LPV by comparison with that of the unagglomerated particles. The smallest grains showed a 4.6% drop in LPV whereas the largest (xe2x88x9220+25) showed a drop of nearly 34% by comparison with the LPV of the 60 grit particles.
The agglomerate grains generally have a diameter, (defined as the size of the aperture in a sieve (of series of standard sieves) with the coarsest mesh on which the grains are retained), that is at least two times the diameter of the individual abrasive particles contained therein. The shape of the agglomerate abrasive grains is not critical and they can therefore be random somewhat blocky shapes or, more preferably, somewhat elongated shapes. They can also have an imposed shape this is often advantageous for some applications.
The abrasive particles present in the agglomerates of the invention may include one or more of the abrasives known for use in abrasive tools, such as aluminas, including fused alumina, sintered and sol gel sintered alumina, sintered bauxite, and the like, silicon carbide, alumina-zirconia, garnet, flint, diamond, including natural and synthetic diamond, cubic boron nitride (CBN), and combinations thereof. Any size or shape of abrasive particle may be used. For example, the grain may include elongated sintered sol gel alumina particles having a high aspect ratio of the type disclosed in U.S. Pat. No. 5,129,919 or the filamentary shaped abrasive particles described in U.S. Pat. No. 5,009,676.
The abrasive particles can comprise blends of abrasives of different qualities since often the performance of a premium quality particles is only marginally diminished by dilution with minor amounts of inferior particles. It is also possible to blend the abrasive particles with minor amounts of non-abrasive materials such as grinding aids, pore formers and filler materials of conventional sorts.
Particle sizes suitable for use herein range from regular abrasive grits (e.g., 60 to 7,000 micrometers) to microabrasive grits (e.g., 2 to 60 micrometers), and mixtures of these sizes. For any given abrasive grinding operation, it is generally preferred to use an agglomerate grain with a grit size smaller than a conventional abrasive grain (non-agglomerated) grit size normally selected for this abrasive grinding operation. For example, when using agglomerate grains, 80 grit size is substituted for 54 grit conventional abrasive, 100 grit for 60 grit abrasive and 120 grit for 80 grit abrasive and so on.
The abrasive particles within the agglomerate are bonded together by a metallic, organic or vitreous bond material and these are referred to generically as xe2x80x9cparticle bindersxe2x80x9d.
Particle binders useful in making the agglomerates include vitreous materials, (defined herein to include both conventional glass materials as well as glass-ceramic materials), preferably of the sort used as bond systems for vitrified bonded abrasive tools. These may be a pre-fired glass ground into a powder (a frit), or a mixture of various raw materials such as clay, feldspar, lime, borax, and soda, or a combination of fritted and raw materials. Such materials fuse and form a liquid glass phase at temperatures ranging from about 500 to 1400xc2x0 C. and wet the surface of the abrasive particles and flow to points of closest contact between adjacent particles to create bond posts upon cooling, thus holding the abrasive particles within a composite structure. The particle binder is used in powdered form and may be added to a liquid vehicle to insure a uniform, homogeneous mixture of coating with abrasive particles during manufacture of the agglomerate grains.
Temporary organic binders are preferably added to powdered inorganic coating components, whether fritted or raw, as molding or processing aids. These binders may include dextrins, starch, animal protein glue, and other types of glue; a liquid component, such as water or ethylene glycol, viscosity or pH modifiers; and mixing aids. Use of such temporary binders improves agglomerate uniformity and the structural quality of the pre-fired or green agglomerates. Because the organic binders burn off during firing of the agglomerate grains, they do not become part of the finished grain.
An inorganic adhesion promoter, such as phosphoric acid, may be added to the mixture to improve adhesion of the particle binder to the abrasive particles as needed. The addition of phosphoric acid to alumina grains greatly improves the mix quality when the particle binder comprises a fritted glass. The inorganic adhesion promoter may be used with or without an organic particle binder in preparing the agglomerate grains.
The preferred particle binder is an inorganic material such as a vitreous bond material. This has a distinct advantage over organic particle binders because it permits the agglomerate grains to be deposited on a substrate in the formation of a coated abrasive using a UP technique. The UP deposition technique is also very suited to use when the particles are bonded together using a metallic binder. Since this process is somewhat more effective and controllable than a gravity deposition technique this represents a significant advance over conventional aggregate grains made using an organic resin binder matrix.
The particle binder can also be an organic binder such as a thermosetting resin such as a phenolic resin, an epoxy resin, a urea/formaldehyde resin, or a radiation-curable resin such as an acrylate, a urethane/acrylate, an epoxy-acrylate, a polyester-acrylate and the like. In general thermosetting resins are preferred as organic binders.
The particle binder is present at about 2 to 25 volume %, more preferably 3 to 15 volume %, and most preferably 3 to 10 volume % based on the combined volume of the particles and binder.
It is also foreseen that the particle binder component can be eliminated altogether if the abrasive particles are caused to sinter together in a controlled fashion such that, by material transport between contacting particles, the bond-posts would be autogenously generated. Alternatively where the abrasive particles are alumina, these could be mixed with a sol of relatively small amount of an alpha alumina precursor such as boehmite. Upon firing this would convert to the alpha phase and would serve the same function as bond posts by connecting adjacent particles.
The invention includes coated abrasives incorporating agglomerated abrasive grain wherein the grains are made by a process which comprises the steps of:
a) feeding abrasive particles and a particle binder material, selected from the group consisting essentially of vitrified bond materials, vitrified materials, ceramic materials, inorganic binders, organic binders, water, solvent and combinations thereof, into a rotary calcination kiln at a controlled feed rate;
b) rotating the kiln at a controlled speed;
c) heating the mixture at a heating rate determined by the feed rate and the speed of the kiln to temperatures from about 145 to 1,300xc2x0 C.,
d) tumbling the particles and the particle binder in the kiln until the binder adheres to the particles and a plurality of the particles adhere together to create sintered agglomerate grains; and
e) recovering the sintered agglomerates from the kiln,
whereby the sintered agglomerate grains have an initial three-dimensional shape, a loose packing volume that is at least 2% below the corresponding loose pack volume of the constituent particles and comprise a plurality of abrasive particles.
The invention also includes coated abrasives incorporating sintered abrasive agglomerate grains that have been made by a method comprising the steps:
a) feeding abrasive particles along with a particle binder material into a rotary calcination kiln at a controlled feed rate;
b) rotating the kiln at a controlled speed;
c) heating the mixture at a heating rate determined by the feed rate and the speed of the kiln to temperatures from about 145 to 1,300xc2x0 C.,
d) tumbling the abrasive particles and the particle binder in the kiln until the binder adheres to the grain and a plurality of grains adhere together to create sintered abrasive agglomerate grains; and
e) recovering the sintered agglomerate grains from the kiln,
whereby the sintered agglomerate grains have an initial three-dimensional shape, comprise a plurality of particles and have a loose packing volume that is at least 2% below the corresponding loose pack volume of the constituent particles.